Recommended Antenna Reference Designs; Antenna Selection Guide; Active Antenna Reference Design - Quectel LC79D Hardware Design

GNSS Module Series
LC79D Hardware Design

4.2. Recommended Antenna Reference Designs

4.2.1. Antenna Selection Guide

Both active and passive dual-band (L1+L5) GNSS antennas can be used for LC79D module. Passive
antenna is recommended if the antenna can be placed close to the module, for instance, when the distance
between the module and the antenna is less than 1m. Otherwise, please use an active antenna instead,
since the insertion loss of RF cable can decrease CNR of GNSS signal.
CNR is an important factor for GNSS receivers, and it is defined as the ratio of the received modulated
carrier signal power to the received noise power in one Hz bandwidth. The formula of CNR is as below:
CNR = Power of GNSS signal − Thermal Noise − System NF(dB-Hz)
Among all the quantities, "Power of GNSS signal" means GNSS signal level and in practical environment
the signal level at the surface of earth is about -130dBm. "Thermal Noise" is -174dBm/Hz at 290K. To
improve CNR of GNSS signal, a LNA could be added to reduce "System NF".
For "System NF", it is expressed by formula as:
NF = 10 log F (dB)
And "F" denotes noise factor of receiver system:
F = F1 + (F2 − 1)/G1 + (F3 − 1)/(G1 ∙ G2) + ⋯
"F1" denotes first stage noise factor, "G1" denotes first stage gain, etc. This formula indicates that LNA
with enough gain can compensate noise factor behind the LNA. In this case, "System NF" depends mainly
on noise figure of components and traces before first stage LNA plus noise figure of LNA itself. This
explains the need of using an active antenna while antenna connection cable is too long.

4.2.1. Active Antenna Reference Design

The following figure is a typical reference design for active antenna. In this mode, the antenna is powered
by the VCC_RF on which the voltage is 1.8V.
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